Role Of Silica Fume Research Articles

The Role of Silica Fume in Enhancing the Strength and Transport Properties of PET Fiber–Ultra High-Performance Concrete

The Role of Silica Fume in Enhancing the Strength and Transport Properties of PET Fiber–Ultra High-Performance Concrete

Early hydration of C4AF with silica fume and its role on katoite composition.

C4AF is considered the least reactive main clinker phase, but its reactivity may be affected by adding supplementary cementitious materials (SCMs). Pure C4AF was synthesised in a laboratory furnace, and the role of silica fume without gypsum on its early hydration properties was monitored. Burning was carried out in four stages to achieve 99% purity of C4AF. Heat flow development was monitored by isothermal calorimetry over 7 days of hydration at 20°C and 40°C. The role of silica fume on hydrogarnet phase katoite (Ca3Al2(SiO4)3 - x(OH)4 x x = 1.5-3) formation during early hydration was studied. Rapid dissolution of C4AF, formation of metastable C-(A,F)-H and its conversion to C3(A, F)H6 was evidenced by isothermal calorimetry as a large exotherm. Changes in microstructure during early hydration were documented by SE micrographs, EDS point analyses, X-ray mapping and line scans by SEM-EDS. The phase composition was characterised by DTA-TGA and QXRD after 7 days of hydration. The katoite diffraction pattern is similar for the reference sample and sample with silica fume, but substitution in its structure can be revealed by X-ray microanalyses. The composition of katoite is variable due to the various extent of substitution of 4OH- by SiO4 4- due to silica fume.

Effect of silica fume on rheology of slag-fly ash-silica fume-based geopolymer pastes with different activators

This study examined the effect of silica fume contents on the rheological behavior of slag-fly ash-silica fume-based geopolymer pastes by packing density measurement, low-field NMR, zeta potential, dynamic light scattering and FTIR analyses. Incorporating silica fume into sodium hydroxide (SH)-activated geopolymer pastes resulted in the decrease of packing density and the increases of yield stress, plastic viscosity and hysteresis loop curve area. The contrary results were found in the pastes activated by sodium silicate (SS). The water states of geopolymer pastes were influenced by SiO2/Na2O ratio of activator and silica fume dosage. The mobile water content in the sample increased as the silica fume dosage increased, regardless of the SiO2/Na2O ratio of activator. The particle size of the silica species in the activator and interstitial fluids is close to that of silica fume. The incorporation of silica fume resulted in the variation in the silicate structural characteristics of the interstitial fluids. The influence of silica fume on the rheological behaviors of geopolymer systems is strongly governed by the interparticle repulsion or attractiveness and activator nature, opposed to the filler effect and dissolution of silica fume. In SS-activated systems, the repulsion between the silanol groups on the silica fume surface and silicate species adsorbed on the surface of precursors particles contributed to the particle separations, thereby enhancing rheological behaviors. In SH-activated system, the attractiveness between divalent metal cations and silanol groups on the surfaces of precursor and silica fume particles was increased and the particle space was restrained. This study would provide new insights into the role of silica fume in the particle interaction of fresh ultra-high performance geopolymer concrete.

Immobilization of Cu (Ⅱ), Ni (Ⅱ) and Zn (Ⅱ) in silica fume blended Portland cement: Role of silica fume

Silica fume (SF) is extensively used as supplementary cementitious materials to improve the mechanical and durable properties of Portland cement (PC) and thus has great potential to be applied in PC immobilization technique for the treatment of heavy metal contaminants. This research aims to elucidate the influencing mechanism of SF on the immobilization of heavy metal cations (Cu2+, Ni2+ and Zn2+) in SF-PC blended systems. SF accelerates the early hydration kinetics of PC immobilization systems due to the filler effect. This offsets the retardation impacts of Cu2+ and Zn2+ on the early hydration of PC. The reaction of SF with calcium hydroxide also promotes the later hydration. This decreases the capillary pore volume and increases the compressive strength of PC immobilization systems. The addition of SF induces the formation of C-S-H with high Si/Ca ratio and thus introduces additional SiO− sites to bind Cu2+ and Ni2+. This enhances the capacity of C-S-H for the immobilization of Cu2+ and Ni2+. Zn2+ is transformed to Zn(OH)42− in the high alkaline pore solution of PC. High Si/Ca ratio of C-S-H due to the addition of SF reduces the SiOCa+ and (SiOOH)2AlCa+ sites to bind Zn(OH)42−. This declines the immobilization efficiency of Zn in PC.

Role of silica fume on hydration and strength development of ultra-high performance concrete

Silica fume (SF) is the most commonly used ingredient in ultra-high performance concrete (UHPC) due to its inherited properties, which can effectively improve the rheology and mechanical performance. However, investigations with regard to its filler effect and pozzolanic reaction on UHPC are limited. To assess the contribution from SF, a chemically inert filler of titanium dioxide (TD), with a comparable particle size distribution and specific surface area (per unit volume), was selected as a reference ingredient to eliminate the influence of packing effect. Results showed the addition of SF will accerate the early hydation of UHPC and it is controlled by the combined effect of seeding effect and preferential adsorption. Moreover, the SF’s surface characteristic controlling the concentration of adsorption sites provided for hydration product is the key factor influencing the early hydration rather than surface specific area. Furthermore, the enhancement of compressive strength of UHPC after 3 days is attributed to the pozzolanic reaction of SF and its promoting effect on cement hydration degree. The total contribution is less than 20%, and it will enhances with the decrease of W/C ratio.

Comparative study of effect of raw and densified silica fume in the paste, mortar and concrete

The differences of effect of silica fume in paste, mortar and concrete were studied by determining the non-evaporable water content of pastes, measuring the compressive strengths of pastes, mortars and concretes containing 5% and 10% raw silica fume or densified silica fume with water-to-binder ratios (W/B) of 0.29 and 0.24 and investigating the properties of interfacial transition zone between hardened paste and aggregate. The results show that silica fume can significantly increase the hydration degree of paste. The addition of silica fume trends to increasing the compressive strengths of hardened pastes, mortars and concretes, and the strength activity index of densified silica fume in concrete is the highest while that in paste is the lowest. The agglomeration of silica fume has been found in blended paste which is hardly seen in concrete. The silica fume can improve the interface bond strength between hardened cement paste and aggregate. The crystalline orientation degree, the crystalline size and the content of calcium hydroxide at the interface are obviously decreased by adding silica fume. The different dispersion and the improvement of the interfacial transition zone are the main factors causing the different role of silica fume in paste, mortar and concrete.

The Role of Silica Fume in the Adhesion of Concrete Restoration Systems

The weak interfacial transition zone between new and old concrete is always paid much attention and controls many properties of repaired concrete, The present work reports a study on the influence of the silica fume on the adhesion of reactive powder concrete (RPC), as a concrete restoration material, with the ordinary concrete (OC) substrate. The results showed that, the silica fume presence in the interfacial transition zone significantly enhances the adhesion strength between RPC and OC substrate. Furthermore, the silica fume particles consume calcium hydroxide, which is in attendance in the interfacial transition zone, and make the zone more dense, uniform and tough.

Investigation of bond between lap-spliced steel bar and self-compacting concrete: The role of silica fume

In this study, normal concrete (NC) and four types of self-compacting concrete (SCC), in which cement was partially replaced by 5%, 10%, 15%, and 20% of silica fume (SF), were used to evaluate the effect of SF content on the bond strength of tension lap-spliced bars embedded in NC and SCC specimens. Therefore, 15 full-scale beam specimens (2000 × 300 × 200 mm3) were tested and 20 mm reinforcing bars were used with a 300-mm splice length as tension reinforcement. Each beam was designed with bars spliced in a constant moment region at midspan. It was found that the bond strength of the reinforcement embedded in SCC beams was higher than that of the reinforcement in NC beams, whilst the bond strength increased with increase in the replacement of cement by SF from 5% to 10%. Moreover, the beam specimens produced from SCC containing 5% SF had the highest normalized bond strength of 1.07 followed by SCC beams with 10% SF, 15% SF, NC beams, and 20% SF.

Mitigating effect of chloride ions on sulfate attack of cement mortars with or without silica fume

This paper presents a detailed experimental study on the sulfate attack of mortar specimens with or without silica fume exposed to sulfate and sulfate–chloride solutions (with the same concentration of SO42– ions) up to 510 d. The overall aim of the study is to investigate the beneficial effect of chloride ions on sulfate attack. In addition, the role of silica fume and water–binder ratio (w/b) in resisting sulfate attack is also reported. To qualitatively assess the performance of mortar specimens exposed to test solutions, visual examination and compressive strength and expansion tests were carried out. X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) techniques were also used to evaluate the products formed by hydration and chemical reaction and the change of porosity for paste samples. Results indicated that the presence of chloride ions in sulfate environments mitigated the deterioration of ordinary Portland cement mortar specimens, especially with a higher w/b, due to sulfate attack. It seems that the mitigating effect of chloride ions on sulfate attack is attributable to the increased solubility of sulfate products in the chloride-bearing sulfate solution, and the chemical binding of the ions to form Friedel’s salt.

Sulfate attack and role of silica fume in resisting strength loss

Abstract This paper presents a detailed experimental study on the sulfate attack of Portland cement mortars, and the effectiveness of silica fume in controlling the damage arising from such attack. The test solutions used to supply the sulfate ions and cations were 5% sodium sulfate solution and 5% magnesium sulfate solution. Tap water was used as the reference solution. The main variables investigated in the study were the water/cementitious materials ratio, and the level of cement replacement. Compressive strength measured on 50 mm cubes was used to assess the changes in the mechanical properties of mortar specimens exposed to sulfate attack for 510 days. X-ray diffraction and differential scanning calorimetry were used to evaluate the microstructural nature of the sulfate attack. The test results showed that the presence of silica fume had a beneficial effect on the strength loss due to sodium sulfate attack. The best resistance to sodium sulfate attack was obtained with a SF replacement of 5–10%, but even then, a strength loss of 15–20% can be expected. On the other hand, mortars with silica fume were severely damaged in the magnesium sulfate environment. Further, the compressive strength loss actually increased with increasing SF content. The test results thus showed clearly that the use of SF in concrete exposed to magnesium sulfate solution is not recommended. The test results also showed that the w/cm ratio is the most critical parameter influencing the resistance of concrete to sulfate attack. All the tests reported in the study were carried out at 20 ± 1 °C.

The role of silica fume in the kinetics and mechanisms during the early stage of cement hydration

The role of silica fume in the reaction kinetics and mechanisms of the early stage hydration of Portland-slag cement–silica fume pastes (W/S=0.5 at 20°C) has been studied. Using differential scanning calorimetry (DSC), the rate of calcium hydroxide (CH) formed during hydration has been determined quantitatively and the percentage of reaction of hydration has been calculated. The kinetic analysis was used to obtain parameters, which were employed to find out the influence of the silica fume content on the reaction rate constant value. The reaction stages have been analysed and explained by kinetic terms. The delay in CH formed during hydration between 8 and 10 h from the beginning of hydration has been noticed and a mechanism has also been discussed. The investigations have shown that the addition of silica fume of more than 8 mass% reduces the duration of the phase boundary interaction as a rate-determined process resulting in the fast diffusion rate-determining process. The results of this study have also revealed evidence of the accelerator effect of silica fume during the first 8 h of hydration when it still exists as chemically inert filler. The pozzolanic reaction between silica fume and CH formed during hydration is occurring after three days of hydration.

The role of silica fume in the direct tensile strength of cement-based materials

The direct tensile strength of silica fume cement paste and mortar were evaluated at various water-cementitious content ratios. Four different water-cementitious content ratios of 0.22, 0.25, 0.28, and 0.31 were used, and three contents of silica fume, 8%, 16%, and 25% by mass of cement. Superplasticizer content was adjusted for each mix to ensure that no segregation would occur.

Bond Strength of Reinforcement in High Performance Concrete: Role of Silica Fume, Casting Position, and Superplasticizer Dosage

Little research is reported in the literature on the effect of pozzolans such as silica fume on structural behavior of reinforced concrete, namely on bond and anchorage characteristics of reinforcing bars in concrete. The objectives of the study were to investigate the effect of silica fume on bond and anchorage characteristics of reinforcing bars in high performance concrete, to study the validity of the upper limit of 70 MPa (10,000 psi) imposed by the American Concrete Institute (ACI) Building Code 318-95 on the concrete compressive strength for determination of development length, and to evaluate the reliability of the empirical equation of Orangun, Jirsa, and Breen in estimating the bond strength of deformed bars embedded in high strength concrete. Sixteen beam specimens were tested. Each beam was designed to include two bars in tension, spliced at the center of the span. The splice length was selected so that bars would fail in bond, splitting the concrete cover in the splice region, before reaching the yield point. The beams were loaded in positive bending with the splice in a constant moment region. The variables used were the percentage replacement by weight of cement by silica fume, casting position, and the superplasticizer dosage. Test results indicated that replacement of 5-20% of the cement by an equal weight of silica fume resulted in an average 10% reduction in bond strength regardless of casting position or the superplasticizer dosage used.

Competitive nature of alkali-silica fume and alkali–aggregate (silica) reaction

Silica fume is an effective pozzolan in controlling alkali-aggregate (silica) reactionwhen used in appropriate amounts as a cement replacement. The experiments described in this Paper showed that silica fume reacts in a similar way to alkali-expansive opal with CaOH)2 and/or NaOH. Pastes of silica fume and Ca(OH)2, silica fume and NaOH, and silica fume and Ca(OH)2 plus NaOH showed the same expansive properties as corresponding pastes made with opal and these three alkalis. The role of silica fume in controlling alkali-aggregate (silica) reaction is attributed to a competitive reaction between alkali-silica fume and alkali-aggregate (silica) reactions.

Role of Silica Fume in Compressive Strength of Cement Paste, Mortar, and Concrete

Controversy exists as to why silica fume increases the strength of concrete when it is used as a partial replacement for cement. Some evidence supports the view that the increase in strength is due to an increase in the strength of the cement paste constituent of concrete. However, contradictory evidence exists that shows no increase in the strength of cement paste, but substantial increases in concrete strength, when silica fume is used. The latter evidence is used to support the theory that silica fume strengthens concrete by strengthening the bond between cement paste and aggregate. This study is designed to explain the contradictory evidence and establish the role played by silica fume in controlling the strength of concrete and its constituent materials. These goals are accomplished using cement pastes, mortars, and concretes with water-cementitious material ratios ranging from 0.30 to 0.39. Mixtures incorporate no admixtures, a superplasticizer only, or silica fume and a superplasticizer. The research demonstrates that replacement of cement by silica fume and the addition of a superplasticizer increases the strength of cement paste. It also demonstrates that cement paste specimens, with or without silica fume, can exhibit reduced strength compared to other specimens with the same water-cementitious material ratio if the material segregates during fabrication, thus explaining some earlier experimental observations. The segregation of cement paste is caused by high superplasticizer dosages that do not cause segregation of concrete with the same water-cementitious material ratio. Concrete containing silica fume as a partial replacement for cement exhibits an increased compressive strength because of the improved strength of its cement paste constituent. Changes in the paste-aggregate interface caused by silica fume appear to have little effect on the uniaxial compressive strength of concrete.